docs: def check_duplicate_axis(self, ds):
          '''
          Checks that no variable contains two coordinates defining the same
          axis.
  
          Chapter 5 paragraph 6
  
          If an axis attribute is attached to an auxiliary coordinate variable,
          it can be used by applications in the same way the `axis` attribute
          attached to a coordinate variable is used. However, it is not
          permissible for a [geophysical variable] to have both a coordinate
          variable and an auxiliary coordinate variable, or more than one of
          either type of variable, having an `axis` attribute with any given
          value e.g. there must be no more than one axis attribute for X for any
          [geophysical variable].
  
          :param netCDF4.Dataset ds: An open netCDF dataset
          :rtype: compliance_checker.base.Result
          :return: List of results
          '''
  
          ret_val = []
          geophysical_variables = self._find_geophysical_vars(ds)
          for name in geophysical_variables:
              no_duplicates = TestCtx(BaseCheck.HIGH, self.section_titles['5'])
              axis_map = cfutil.get_axis_map(ds, name)
              axes = []
              # For every coordinate associated with this variable, keep track of
              # which coordinates define an axis and assert that there are no
              # duplicate axis attributes defined in the set of associated
              # coordinates.
              for axis, coordinates in axis_map.items():
                  for coordinate in coordinates:
                      axis_attr = getattr(ds.variables[coordinate], 'axis', None)
                      no_duplicates.assert_true(axis_attr is None or axis_attr not in axes,
                                                "'{}' has duplicate axis {} defined by {}".format(name, axis_attr, coordinate))
  
                      if axis_attr and axis_attr not in axes:
                          axes.append(axis_attr)
  
              ret_val.append(no_duplicates.to_result())
  
          return ret_val